Membrane or miniature hydrophone probes are used to determine the properties of an ultrasonic field prevailing in a sound-carrying medium such as water. The three-dimensional distribution of the ultrasonic field is sound pressure amplitude from the ultrasonic field is determined by measuring the prevailing sound pressure in a measuring tray at different locations using a hydrophone probe.
In "Ultrasonics," May 1980, pages 123-126, a membrane hydrophone is disclosed. The membrane hydrophone uses a foil of polyvinylidene fluoride PVDF having a thickness of 25 .mu.m. The PVDF foil is stretched between two metal rings serving as support bodies to form a diaphragm having an inside diameter of about 100 mm. Each surface of the membrane includes a small central region having oppositely disposed circular disk-like electrodes. The diameter of the disk-like electrodes are, for example, 4 mm. A polarized, piezo-electrically induced active region is between these electrodes. Metal film leads are applied to the diaphragm surfaces to carry the signals from the circular disk-like active region of the diaphragm to the diaphragm rim. The metal film leads are coupled at the diaphragm rim to a coaxial cable by means of a conductive adhesive.
It is not possible, however, to measure with the prior hydrophones, ultrasonic shock waves having pressure amplitudes in the range of 10.sup.8 Pa. Such shock waves have very steep pulse flanks. The rise times of the steep pulse flanks are less than 1 usec which leads to mechanical destruction of the hydrophones. The mechanical destruction is caused by the cavitation effects of the metallic electrodes that are applied to the PVDF layer. These high pressure amplitude shock waves occur, for example, in the focal range of lithotripterns in which a focused ultrasonic shock wave is used for destroying concrements such as kidney stones in a patient's kidney. During the development and routine monitoring of hydrophone equipment, it is necessary to determine the properties of the shock wave in the focal range.
European Pat. No. A2-0 227 985 discloses an ultrasound sensor having a polymer foil fastened at its rim to a support body. The polymer film is piezoelectrically activated in a subzone and is electrically coupled to an electrode. The electrode must be arranged physically apart from the piezoelectrically active region. Ultrasonic waves in the piezoelectrically active region of the polymer foil cause surface charge vibrations. These vibrations are electrically coupled via the sound-carrying medium surrounding the polymer foil to the electrodes arranged outside the surface region of the polymer foil related to the piezoelectrically active region. Because no mechanically astable electrically conductive layer is present in the central region of the polymer foil, the piezoelectrically active region can thus be arranged within the focal range of a focused ultrasonic shock wave.
Through the use of a piezoelectric polymer having a small dielectric constant relative to piezoceramic materials, purely capacitive coupling is possible without great signal losses. Accordingly, the electrodes can be physically arranged separate from the piezoelectrically active regions of the polymer foil on the foil itself, as well as outside the foil i.e., at the foil support body.
In this prior device, the piezoelectric polymer foil is tautly clamped between two annular support bodies. The flat surfaces of the polymer foil are oriented perpendicular to the central axis of the support bodies. The ultrasound to be measured has a direction of incidence substantially parallel to the central axis. In order to avoid any interfering diffraction effects on the ultrasound sensor occurring at the inner edge facing away from the center of the polymer foil, the diameter of the polymer foil must be very large. In the prior ultrasound sensor, therefore, miniaturization is always accompanied by a degradation of the receiving properties.